Implantable neurostimulation systems have proven therapeutic in a wide variety of diseases and disorders. Pacemakers and Implantable Cardiac Defibrillators (ICDs) have proven highly effective in the treatment of a number of cardiac conditions (e.g., arrhythmias). Spinal Cord Stimulation (SCS) systems have long been accepted as a therapeutic modality for the treatment of chronic pain syndromes, and the application of tissue stimulation has begun to expand to additional applications, such as angina pectoris and incontinence. Further, in recent investigations, Peripheral Nerve Stimulation (PNS) systems have demonstrated efficacy in the treatment of chronic pain syndromes and incontinence, and a number of additional applications are currently under investigation. More pertinent to the present inventions described herein, Deep Brain Stimulation (DBS) has been applied therapeutically for well over a decade for the treatment of neurological disorders, including Parkinson's Disease, essential tremor, dystonia, and epilepsy, to name but a few. Further details discussing the treatment of diseases using DBS are disclosed in U.S. Pat. Nos. 6,845,267, 6,845,267, and 6,950,707, which are expressly incorporated herein by reference.
Each of these implantable neurostimulation systems typically includes one or more electrode carrying stimulation leads, which are implanted at the desired stimulation site, and a neurostimulator implanted remotely from the stimulation site, but coupled either directly to the neurostimulation lead(s) or indirectly to the neurostimulation lead(s) via a lead extension. A single stimulation lead may contain electrodes of different sizes. The neurostimulation system may further comprise a handheld external control device to remotely instruct the neurostimulator to generate electrical stimulation pulses in accordance with selected electrical stimulation parameters.
Electrical stimulation energy may be delivered from the neurostimulator to the electrodes in the form of an electrical pulsed waveform. Thus, the stimulation energy may be controllably delivered to the electrodes to stimulate the tissue. The combination of electrodes used to deliver the electrical pulses to the targeted tissue constitutes an electrode combination, with the electrodes capable of being selectively programmed to act as anodes (positive), cathodes (negative), and/or left off (zero). In other words, an electrode combination represents the polarity being positive, negative, or zero. Other parameters that may be controlled or varied include the amplitude, width, and rate of the electrical pulses provided through the electrode array. Each electrode combination, along with its electrical pulse parameters, can be referred to as a “stimulation parameter set.”
With some neurostimulation systems, and in particular, those with independently controlled current and/or voltage sources, the distribution of the current to the electrodes (including the case of the neurostimulator, which may act as an electrode) may be varied such that the current is supplied via numerous different electrode configurations. In different configurations, the electrodes may provide current or voltage in different relative percentages of positive and negative current or voltage to create different electrical current distributions (i.e. fractionalized electrode combinations).
As briefly discussed above, an external control device can be used to instruct the neurostimulator to generate electrical stimulation pulses in accordance with selected stimulation parameters. Typically, the stimulation parameters programmed into the neurostimulator can be adjusted by the user by manipulating controls on the external user control device to modify the electrical stimulation provided by the neurostimulator system to the patient. Thus, in accordance with the stimulation parameters programmed by the external control device, electrical pulses can be delivered from the neurostimulator to the stimulation electrode(s) to stimulate or activate a volume of tissue in accordance with the set of stimulation parameters and provide the desired efficacious therapy to the patient. The best stimulus parameter set will typically be one that delivers stimulation energy to the volume of tissue that must be stimulated in order to provide the therapeutic benefit (e.g., treatment of pain), while minimizing the amount of non-target tissue that is stimulated. A typical stimulation parameter set may include the electrodes that acting as anodes or cathodes, as well as the amplitude, duration, and rate of the stimulation pulses.
To facilitate the selection of the stimulation parameters, the clinician generally programs the external control device, and if applicable the neurostimulator, through a computerized programming system. This programming system can be a self-contained hardware/software system, or can be defined predominately by software that is run on a standard personal computer (PC). The PC or custom hardware may actively control the characteristics of the electrical stimulation generated by the neurostimulator to allow the optimum stimulation parameters to be determined based on patient feedback, or other means, and to subsequently program the external control device with the optimum electrical stimulation parameters.
When electrical leads are implanted within the patient, the computerized programming system may be used to instruct the neurostimulator to apply electrical stimulation to test placement of the leads and/or electrodes, thereby assuring that the leads and/or electrodes are implanted in effective locations within the patient. Once the leads are correctly positioned, a fitting procedure, which may be referred to as a navigation session, may be performed using the computerized programming system to program the external control device, and if applicable the neurostimulator, with a set of stimulation parameters that best addresses the disorder or painful site.
Significantly, there are limits to how much charge (both in terms of total charge per pulse (or phase) and charge density per pulse) can be injected into tissue without causing cell trauma and/or electrochemical damage (i.e., corrosion) to the electrodes. Each electrode, depending upon its physical properties (which include, but are not limited to, its size, shape, and material), has a charge threshold level that should not be exceeded to ensure that the amount of charge applied to the electrode will not cause irreparable electrochemical harm to the electrode. Smaller sized electrodes generally have lower charge threshold levels than larger sized electrodes that are manufactured of the same material because the smaller sized electrodes have higher charge densities.
With regard to tissue safety, both total charge and charge density should be taken into account to avoid cell trauma. As such, the Shannon Model was created in 1992 for evaluating tissue safety limits using k-values (k=log(Total Charge)+log(Charge Density)). (See Shannon, R. V., A Model of Safe Levels for Electrical Stimulation, IEEE-TBME, Vol. 39, No. 4, pp. 424-426, April 1992). The Shannon model indicates that a tissue safety limit of k equal to 1.5 or lower should be maintained to ensure tissue safety.
Previously, a patient undergoing neurostimulation therapy would exhibit side effects well before cell trauma would occur. The onset of side-effects is primarily caused by the total charge per pulse (as well as the charge density per pulse), thereby naturally limiting the total charge per pulse that can be applied to the patient. However, due to the relatively large area, and resulting low charge density, of prior art electrodes, the charge density per pulse was also naturally limited by the side-effects experienced by the patient. However, as the size of electrodes becomes smaller (e.g., the use of segmented electrodes is becoming prevalent in the context of DBS), thereby effectively increasing the charge density per pulse, it is possible to cause cell trauma before the onset of side-effects. It is known to incorporate hard stop limits into neurostimulation systems to ensure that the total charge per pulse injected into the tissue region is within a tissue safety limit. However, these hard stop limits do not take into account charge density per pulse.
Some conventional neurostimulation systems also have warning thresholds that are configured to notify the user that the charge injected into the tissue is at or above a warning threshold by displaying a simple textual notification message. Unlike hard stop limits, these warning thresholds do not necessarily prevent the user from meeting or exceeding the defined safety limit value for tissue charge injection. While, in many cases, the use of warning safety limits for tissue charge injection may warn the user in enough time before tissue damage (and/or electrode damage) occurs, in other cases, the user may desire to have knowledge of the amount of tissue charge injection well before the safety warning threshold is reached.
In addition, most current conventional implantable neurostimulation systems only globally monitor and control the amount of charge that is injected into the tissue. That is, only one electrode (e.g., the worst-case electrode) is monitored, and based on this, the tissue charged injected by all of the electrodes is globally controlled. However, in most cases, the tissue charge injection associated with each active electrode may substantially differ. This is especially the case if different sized electrodes are being used. This is because smaller sized electrodes inherently have a higher charge density than larger sized electrodes, thereby increasing the possibility of tissue damage occurring at these smaller electrodes. Since these conventional neurostimulation systems can only globally monitor and control the tissue charge injection associated with the electrodes, there is a danger that the smaller electrodes may produce stimulation with a charge density high enough to cause damage to the tissue if the tissue charge injection if the warning threshold is set based on a larger electrode, or there is the possibility that the user will be alerted with a notification message too soon if the warning threshold is set based on a smaller electrode.
Additionally, some conventional neurostimulation systems are designed to “steer” electrical current between electrodes in order to move the resultant stimulation region in a defined direction. Typically, these systems proportion the current across the electrodes at various predefined different percentages over time (i.e. steer the current across the electrodes). For example, a system may displace the electrical stimulation energy along the tissue region by incrementally shifting the electrical current from a first group of electrodes to a second group of electrodes, and then from the second group of electrodes to a third group of electrodes, and so on. These systems steer the current without regard to the amount of charge injected within tissue for each incremental shift in the electrical current, thereby posing an increased risk that the charge injected by any particular electrode will damage the tissue, or if one exists, the user may steer the current into the warning limit, thereby reducing the usability of the system.